This invention relates to machines for making plastic bags, and more particularly to improved means for mounting a bag punching and slitting device on such machines. Even more particularly this invention relates to improved means of the type described which utilizes a single fluid pressure-operated cylinder for simultaneously gripping, punching and slitting plastic bags made on a machine of the type described.
In machines of the type described for manufacturing plastic bags, overlapping webs of thermoplastic material are passed beneath one or more punching and slitting devices, which function simultaneously to form adjacent slits and circular openings or perforations into what is to be the open end of a respective bag. For example, in my co-pending U.S. patent application Ser. No. 07/529,646, which was filed May 29, 1990, and which issued on Jul. 30, 1991 as U.S. Pat. No. 5,035,165, I have disclosed a combination punching and slitting device which is adapted to be mounted on a machine of the type described for operation by a fluid pressure-operated cylinder, such as for example a compressed air cylinder. Heretofore in a machine of this type it has been customary to use one fluid pressure-operated cylinder for simultaneously reciprocating the punch and blade device toward and away from the overlapping plastic webs, and a separate fluid pressure-operated cylinder for manipulating a gripping device which holds the overlapping webs in a stationary position during the bag punching and slitting operation.
FIG. 2 of the U.S. Pat. No. 3,802,308, for example, illustrates a first fluid pressure-operated cylinder 52 that is used, among other things, to reciprocate a pair of web retainers which grip and hold in place a pair of overlapping, plastic sheets or webs, at the same time that a second such cylinder (88) reciprocates a combination punching and slitting device toward and away from the now-gripped webs.
A primary disadvantage of prior such web gripping and punching/slitting mechanisms is that at least two separate fluid pressure-operated cylinders are required to operate, respectively, the web gripping members, and the punching/slitting device. In addition to the increased costs required by duplicating the fluid pressure-operated cylinders, there is also the care and control that must be exercised in order to assure that the respective cylinders and associated mechanisms are operated in proper sequence. The need for carefully gripping and holding plastic sheets or films as they are being punched and slit is particularly important in the case of very thin films or plastic sheets, which otherwise exhibit undesirable stretching of the film or sheet around the hole which is punched. It is essential also that tough, high density plastic materials also be carefully clamped during the punching operation to achieve maximum punch and blade life.
It is an object of this invention, therefore, to provide an improved bag punching and slitting device that incorporates therein a web gripping device, and which utilizes a single, fluid pressure-operated cylinder for operating both the punch/slitter device and the associated web gripping mechanism.
Still another object of this invention is to provide an improved bag punching and slitting mechanism which, immediately upon operation of the associated bag punching and slitting device, operatively grips and holds the overlapping webs or films that are to be punched and slit.
A further object of this invention is to provide a combined punch/slitter and web gripping mechanism of the type described which can be quickly mounted upon or removed from an associated bag making machine to effect adjustment or replacement thereof.
Other objects of the invention will be apparent hereinafter from the specification and from the recital of the appended claims, particularly when read in conjunction with the accompanying drawings.